1. Field of the Invention
The present invention relates to a method and apparatus for erasing acoustic echo.
2. Description of the Related Art
In communications apparatuses having a speech amplifying function for use in hands-free telephones, TV conference communications apparatuses and so on, an acoustic echo canceler is well known for canceling acoustic echo possibly generated on the basis of sound radiated from a speaker forming part of a receiver. Specifically, in communications between two speaker sides, sound radiated on one speaker side travels as an input signal to a microphone forming part of a transmitter of the one speaker side and then is included as acoustic echo into sound received by the other speaker side.
The conventional acoustic echo canceler, however, is not capable of appropriately canceling acoustic echo if so-called double talk occurs, where a feedback signal generated by sound radiated from a speaker of one speaker side and a voice transmission signal (also including surrounding sound) of the one speaker side are mixedly input into a microphone of the one speaker side. More specifically, the conventional echo canceler basically generates pseudo-echo for performing waveform processing on an input voice signal to remove echo included therein, and adds the pseudo-echo to an echo return path in opposite phase to erase the echo. Thus, when double talk occurs, the conventional acoustic echo canceler also recognizes a voice transmission signal of the one speaker side as an echo component to generate erroneous pseudo-echo, thereby occasionally failing to stably cancel acoustic echo.
An article entitled "Studies on Echo Canceler Having Duo-filter Using ES Projection Algorithm" by Hada and other three, Transactions of Conference held by Institute of Acoustics Engineers of Japan, March 1995, pp. 595-596 discloses a method as countermeasures to the occurrence of double talk as mentioned above.
According to the article, the echo canceler comprises an adaptive filter for estimating a transfer characteristic of an acoustic echo path; a semifixed filter coupled to receive a filter coefficient generated by the adaptive filter to actually cancel echo; and a convergence state determination circuit for controlling to transfer the adaptive filter coefficient to the semifixed filter when an error of the adaptive filter coefficient is small and when a comparison between the error of the adaptive filter coefficient and a coefficient generated by the semifixed filter determines that the error of the adaptive filter coefficient is smaller. Since the convergent state determination circuit prohibits the transfer of the filter coefficient from the adaptive filter to the semifixed filter when it detects disturbance in the adaptive filter coefficient caused by the occurrence of double talk, the semifixed filter holds a filter coefficient estimated immediately before the double talk occurs. In this way, favorable echo erasure can be carried out event when double talk occurs. Stated another way, since a voice transmission signal during double talk is disturbance added to a transfer path from a loud speaker to a microphone and does not cause any change in the characteristic of the transfer path from the loud speaker to the microphone, a coefficient generated before the disturbance occurs is used to erase echo so that the disturbance, i.e., the voice transmission signal only can be separated.
However, the method disclosed in this article essentially requires the convergence state determination circuit for detecting a disturbed filter coefficient to take countermeasures to double talk and the semifixed filter for holding a filter coefficient in a steady state, an apparatus implementing the method becomes excessively complicated.
Generally, simplification is strongly required for acoustic echo cancelers for use in hands-free telephones and so on, so that further improvements are desired to such acoustic echo cancelers.